The Lapstone structural complex, New South Wales

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Abstract
The Lapstone Structural Complex forms the most prominent topographic feature in the Sydney region. The complex consists of a number of related folds and faults, trending generally north‐south, which together form a large south‐plunging structure between Kurrajong Heights and Lapstone. The east‐facing escarpment of the Blue Mountains, formerly called the Lapstone Monocline, varies in its character, being sometimes a single monocline, sometimes a double monocline and sometimes a normal or high‐angle reverse fault. Faulting west of Kurrajong and at Glenbrook is part of a series of overlapping en echelon faults, west‐side down, and sometimes overturned, rather than a single fault. This fault system forms the west side of the complex. Significant minor structures associated with major features include thrusts, minor folds, joint systems, tectonic breccias, sedimentary injections and igneous dykes. Many of the minor structures show a marked parallelism with the major structures. The main period of deformation forming the complex is believed to have taken place in the Early Tertiary, but the overall structure has a long and complex history. Field evidence suggests that sinistral strike‐slip faulting played a part in the deformation, particularly of the near‐surface rocks. Basement block faulting was also significant, producing the Cumberland Basin and associated structures when the main Lapstone structures were formed. Basement structural control is believed to consist of the northerly extension of the western edge of the Eden‐Comerong‐Yalwal Rift intersected by elements of the east‐trending Lachlan Lineament.

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